1. Field of the Invention
This invention relates to a device for driving an elevator at the time of a service interruption which is driven by a PWM inverter and more particularly to a device for driving an elevator at the time of a service interruption wherein a regenerative power consumption circuit is deleted to achieve cost reduction.
2. Description of the Related Art
Recently, a PWM inverter has been used for controlling an induction motor for driving an elevator in order to achieve purposes such as the reduction of power consumption. In the case of this type of elevator control device, since the PWM inverter is driven by the power supplied by a battery at the time of a service interruption, there is an advantage that it is not necessary to provide a complicated inverter controller, insulated DC-DC converter, or constant voltage circuit. For example, FIG. 5 illustrates the configuration of a conventional device for driving an elevator at the time of a service interruption disclosed in Japanese Patent Publication No. 64-314.
In the FIG. 1 represents a three-phase AC commercial power source; 2 represents normally open contacts of a contactor which is connected to the commercial power source 1 and excited when an elevator is running; 3 represents a PWM converter (hereinafter simply referred to as converter) converting the three-phase AC voltage of the commerical power source 1 into DC voltage; 4 represents a capacitor for smoothing the output voltage of the converter 3; 5 represents a regular PWM inverter (hereinafter referred to as regular inverter) for converting the DC voltage across the capacitor 4 into VVVF (variable voltage variable frequency) three-phase AC voltage; and 6 represents an induction motor for driving the elevator driven by the regular inverter 5.
7 represents a control circuit for controlling transistors in the regular inverter 5 in response to three-phase voltage command values V.sub.lU, V.sub.lV, and V.sub.lW. 8 represents a velocity detector for detecting the rotor angular velocity .omega.r of the induction motor 6. 9 represents current detectors for detecting three-phase AC currents 1.sub.lU, 1.sub.lV, and 1.sub.lW flowing from the regular inverter 5 to the stator (primary winding) of the induction motor 6.
10 represents a resistor connected across the capacitor 4. 11 represents a transistor which is series-connected to the resistor 10 to be controlled by the control circuit 7. The series circuit comprising the resistor 10 and transistor 11 constitutes a regenerative power consumption circuit for consuming the regenerative power generated by the induction motor 6 at the time of a service interruption. 13 represents a three-phase transformer which supplies power to the control circuit 7 and is connected to the commercial power source 1 at the primary side thereof.
14 represents a battery for supplying power to the regular inverter 5 and control circuit 7 at the time of an interruption at the commercial power source 1. 15 represents normally open contacts of a contactor which is connected to the battery 14 and is excited at the time of a service interruption. 16 represents a diode for supplying the regular inverter 5 with the DC power of the battery 14. 17 represents an inverter for service interruption which converts the DC voltage from the battery 14 into three-phase AC voltage at the time of a service interruption.
18 represents normally open contacts inserted between the inverter for service interruption 17 and the primary of the three-phase transformer 13. 19 represents normally closed contacts inserted between the commercial power source 1 and the primary of the three-phase transformer 13. The normally open contacts 18 and normally closed contacts 19 cooperate with a relay excited when the commercial power source 1 is interrupted, constituting a switching means for switching from the commercial power source 1 to the inverter for service interruption 17 to connect the latter to the primary of the three-phase transformer 13.
FIG. 6 is a block diagram showing the control circuit 7 in FIG. 5 in detail.
21 represents a three-phase-to-two-phase converter connected to the current detectors 9, for converting the three-phase AC currents 1.sub.lU - 1.sub.lW into primary currents in a two-axis (d-q) rotary coordinate system, i.e., a d-axis current I.sub.l d corresponding to an exciting current and a q-axis current I.sub.l q corresponding to a torque current according to an AC frequency .omega..sub.l.
22 represents a magnetic flux computing unit connected to the three-phase-to-two-phase converter 21 for computing a d-axis magnetic flux .phi..sub.2 d which interlinks with the rotor (secondary) side according to the currents I.sub.l d and I.sub.l q and voltage command values V.sub.l d and V.sub.l q of the two axes.
23 represents a two-phase-to-three-phase converter for outputting the three-phase voltage command values V.sub.lU, V.sub.lV, and V.sub.lW to the regular inverter 5. It converts the voltage command values V.sub.l d and V.sub.l q in the two-axis rotary coordinate system into the three-phase voltage command values V.sub.lU, V.sub.lV, and V.sub.lW according to the AC frequency .omega..sub.l.
24 represents a d-axis current controller for amplifying the deviation between a d-axis current command value I.sub.lD and the d-axis current I.sub.l d of the primary winding to generate the d-axis voltage command value V.sub.l d. 25 represents a q-axis current controller for amplifying the deviation between a q-axis current command value I.sub.lQ and the q-axis current I.sub.l q to generate the q-axis voltage command value V.sub.l q. 26 represents a magnetic flux controller for amplifying the deviation between a predetermined d-axis magnetic flux command value .phi..sub.2D according to the rating of the induction motor 6 and the d-axis magnetic flux .phi..sub.2 d to generate the d-axis current command value I.sub.lD. 27 represents a velocity controller for amplifying the deviation between a rotor angular velocity command value .omega..sub.R and a rotor angular velocity .omega.r to generate the q-axis current command value I.sub.lQ.
28 represents a divider for obtaining the ratio I.sub.lQ /.phi..sub.2d of the q-axis current command value I.sub.lQ to the d-axis magnetic flux .phi..sub.2d. 29 represents a coefficient unit for multiplying the calculation result of the divider 28 by .alpha. to obtain a slip frequency command value .omega.s.
30 represents a subtracter for obtaining the deviation between the d-axis magnetic flux command value .phi..sub.2D and the d-axis magnetic flux .omega..sub.2 d. 31 represents a subtracter for obtaining the deviation between the rotor angular velocity command value .omega..sub.R and the rotor angular velocity .omega.r. 32 represents a subtracter for obtaining the deviation between the d-axis current command value I.sub.lD and d-axis current I.sub.ld. 33 represents a subtracter for obtaining the deviation between the q-axis current command value I.sub.lQ and the q-axis current I.sub.l q. 34 represents an adder for adding a slip frequency command value .omega.s and the rotor angular velocity .omega.r to generate the AC frequency .omega..sub.l.
Next, the operation of the conventional device for driving an elevator at the time of a service interruption shown in FIG. 5 will be described.
When the commercial power source 1 works properly, the normally open contacts 15 and 18 are open and the normall closed contacts 19 are closed, so the control circuit 7 is fed by the commercial power source 1. When the elevator is running, the normally open contacts 2 are closed. The regular inverter 5 is then fed by the commercial power source 1 through the converter 3. Therefore, the regular inverter 5 outputs three-phase AC power of a voltage and frequency in accordance with the three-phase voltage command values V.sub.lU, V.sub.lV, and V.sub.lW from the control circuit 7 to control the torque and revolution speed of the induction motor 6.
When the induction motor 6 generates regenerative power, the converter 3 regenerates the regenerative power at the side of the commercial power source 1 through the capacitor 4.
On the other hand, at the time of a service interruption of the commercial power source 1, the normally open contacts 15 and 18 are closed by the power of an emergency auxiliary power source (not shown) or the like to open the normally closed contacts 19. As a result, the control circuit 7 is disconnected from the commerical power source 1 and fed by the battery 14 through the inverter for service interruption 17.
At this time, the inverter for service interruption 17 converts the DC voltage from the battery 14 to generate three-phase AC voltage which is applied to the primary of the three-phase transformer 13 to feed the control circuit 7.
The DC voltage output by the battery 14 is also applied to the DC side of the regular inverter 5 through the diode 16.
Therefore, the control circuit 7 controls the regular inverter 5 in the same way as in the case that the commercial power source 1 works properly, and the regular inverter 5 controls the torque and the revolution speed of the induction motor 6.
At the time of a service interruption of the commercial power source 1, even if regenerative power is generated by the induction motor 6, it can not be returned to the commercial power source 1. As a result, there is a possibility that the voltage at the DC side of the regular inverter 5 will be increased and the elements in the regular inverter 5 are broken. Therefore, the control circuit 7 detects the regenerative power and brings the transistor 11 into a conducting state to consume the regenerative power through the resistor 10.
Next, the operation of the conventional control circuit 7 will be described with reference to FIG. 6.
First, the current detector 9 detects the three-phase AC currents I.sub.lU, I.sub.lV, and I.sub.lW flowing from the regular inverter 5 to the primary winding of the induction motor 6.
The three-phase-to-two-phase converter 21 converts the detected three-phase AC currents I.sub.lU, I.sub.lV, and I.sub.lW into the d-axis current I.sub.l d and q-axis current I.sub.l q viewed from the two-axis rotary coordinate system (d-q coordinate system) rotating in synchronism with the frequency .omega..sub.l of the three-phase AC voltage applied to the primary winding of the induction motor 6.
According to the d-axis voltage command value V.sub.l d, the d-axis current controller 24 performs control so that I.sub.l d equals I.sub.lD and a current in accordance with the command value flows. Similarly, according to the q-axis voltage command value V.sub.l q, the q-axis current controller 25 performs control so that I.sub.l q equals I.sub.lQ.
That is, the voltage command values V.sub.l d and V.sub.l q are converted by the two-phase-to-three-phase converter 23 into three-phase voltage command values V.sub.lU, V.sub.lV, and V.sub.lW which are in turn applied to the regular inverter 5 to cause the desired current to flow through the induction motor 6.
On the other hand, the divider 28 and coefficient unit 29 calculate the slip frequency command value .omega.s on the basis of the relationship .omega.s=.alpha..I.sub.lQ /.phi..sub.2 d. The adder 34 adds the slip frequency command value .omega.s and the rotor angular velocity .omega.r to obtain the AC frequency .omega..sub.l of the voltage applied to the primary winding which is input to the three-phase-to-two-phase converter 21 and two-phase-to-three-phase converter 23.
Thus, the two-phase-to-three-phase converter 23 controls the regular inverter 5 so that the voltage with the AC frequency .omega..sub.l is actually applied to the induction motor 6.
As described above, when the induction motor 6 is controlled by the regular inverter 5 according to the control circuit 7 to decelerate and stop the elevator car or to drive it downward with a heavy load, the regenerative power produced by the induction motor 6 is returned to the DC side through the regular inverter 5. Further, when the commercial power source 1 is interrupted, in order to prevent the elements from being broken by the voltage increase at the DC side of the regular inverter 5, the transistor 11 is brought into a conducting state so that the regenerative power is consumed by the resistor 10.
As described above, the conventional device for driving an elevator at the time of a service interruption requires the regenerative power consumption circuit for protecting the elements in the regular inverter 5 as a countermeasure against the regenerative power produced by the induction motor 6 when the commercial power source 1 is interrupted, resulting in the problem of cost increase.